Tuesday, September 27, 2016

A while ago, I "rebranded" my second re-engineered version of npm2nix into node2nix and officially released it as such. My main two reasons for giving the tool a different name is that node2nix is neither a fork nor a continuation of npm2nix, but a tool that is written from scratch (though it incorporates some of npm2nix's ideas and concepts including most of its dependencies).

Furthermore, it approaches the expression generation problem in a fundamentally different way -- whereas npm2nix generates derivations for each package in a dependency tree and composes symlinks to their Nix store paths to allow a package to find its dependencies, node2nix deploys an entire dependency tree in one derivation so that it can more accurately mimic NPM's behaviour including flat-module installations (at the expense of losing the ability to share dependencies among packages and projects).

Because node2nix is conceptually different, I have decided to rename the project so that it can be used alongside the original npm2nix tool that still implements the old generation concepts.

Besides officially releasing node2nix, I have recently extended its feature set with a new concept for a recurring class of NPM development projects.

Global NPM development dependencies

As described in earlier blog posts, node2nix (as well as npm2nix) generate Nix expressions from a set of third party NPM packages (obtained from external sources, such as the NPM registry) or a development project's package.json file.

Although node2nix works fine for most of my development projects, I have noticed that for a recurring class of projects, the auto generation approach is too limited -- some NPM projects may require the presence of globally installed packages and must run additional build steps in order to be deployed properly. A prominent example of such a category of projects are Grunt projects.

Grunt advertises itself as a "The JavaScript Task Runner" and can be used to run all kinds of things, such as code generation, linting, minification etc. The tasks that Grunt carries out are implemented as plugins that must be deployed as a project's development dependencies with the NPM package manager.

(As a sidenote: it is debatable whether Grunt is a tool that NPM developers should use, as NPM itself can also carry out build steps through its script directive, but that discussion is beyond the scope of this blog post).

A Grunt workflow typically looks as follows. Consider an example project, with the following Gruntfile.js:

Then we must install the development dependencies with the NPM package manager:

$ npm install

And finally, we can run the grunt command-line utility to execute our tasks:

$ grunt

"Global dependencies" in Nix

Contrary to NPM, Nix does not support "global dependencies". As a matter of fact, it takes all kinds of precautions to prevent global dependencies to influence the builds that it performs, such as storing all packages in isolation in a so-called Nix store (e.g. /nix/store--grunt-1.0.1 as opposed to storing files in global directories, such as /usr/lib), initially clearing all environment variables (e.g. PATH) and setting these to the Nix store paths of the provided dependencies to allow packages to find them, running builds in chroot environments, etc.

These precautions are taken for a very good reason: purity -- each Nix package stored in the Nix store has a hash prefix that is computed from all involved build-time dependencies.

With pure builds, we know that (for example) if we encounter a build performed on one machine with a specific hash code and a build with an identical hash code on another machine, their build results are identical as well (with some caveats, but in general there are no observable side effects). Pure package builds are a crucial ingredient to make deployments of systems reliable and reproducible.

In Nix, we must always be explicit about the dependencies of a build. When a dependency is unspecified (something that commonly happens with global dependencies), a build will typically fail because it cannot be (implicitly) found. Similarly, when a build has dependencies on packages that would normally have to be installed globally (e.g. non-NPM dependencies), we must now explicitly provide them as a build inputs.

The problem with node2nix is that it automatically generates Nix expressions and that global dependencies cannot be detected automatically, because they are not specified anywhere in a package.json configuration file.

To cope with this limitation, the generated Nix expressions are made overridable, so that any missing dependency can be provided manually. For example, we may want to deploy an NPM package named floomatic from the following JSON file (node-packages.json):

[
"floomatic"
]

We can generate Nix expressions from the above specification, by running:

$ node2nix -i node-packages.json

One of floomatic's dependencies is an NPM package named: native-diff-match-patch that requires the Qt 4.x library and pkgconfig. These two packages are non-NPM package dependencies left undetected by the node2nix generator. In conventional Linux distributions, these packages typically reside in global directories, such as /usr/lib, and can still be implicitly found.

By creating an override expression (named: override.nix), we can inject these missing (global) dependencies ourselves:

With the override expression shown above, we can correctly deploy the floomatic package, by running:

$ nix-build override.nix -A floomatic

Providing supplemental NPM packages to an NPM development project

Similar to non-NPM dependencies, we also need to supply the grunt-cli as an additional dependency to allow a Grunt project build to succeed in a Nix build environment. What makes this process difficult is that grunt-cli is also an NPM package. As a consequence, we need to generate a second set of Nix expressions and propagate their generated package configurations as parameters to the former expression. Although it was already possible to do this, because the Nix language is flexible enough, the process is quite complex, hacky and inconvenient.

In the latest node2nix version, I have automated this workflow -- when generating expressions for a development project, it is now also possible to provide a supplemental package specification. For example, for our trivial Grunt project, we can create the following supplemental JSON file (supplement.json) that provides the grunt-cli:

[
"grunt-cli"
]

We can generate Nix expressions for the development project and supplemental package set, by running:

$ node2nix -d -i package.json --supplement-input supplement.json

Besides providing the grunt-cli as an additional dependency, we also need to run grunt after obtaining all NPM dependencies. With the following wrapper expression (override.nix), we can run the Grunt task runner after all NPM packages have been successfully deployed:

As may be observed in the expression shown above, the postInstall hook is responsible for invoking the grunt command.

With the following command-line instruction, we can use the Nix package manager to deploy our Grunt project:

$ nix-build override.nix -A package

Conclusion

In this blog post, I have explained a recurring limitation of node2nix that makes it difficult to deploy projects having dependencies on NPM packages that (in conventional Linux distributions) are typically installed in global file system locations, such as grunt-cli. Furthermore, I have described a new node2nix feature that provides a solution to this problem.

In addition to Grunt projects, the solution described in this blog post is relevant for other tools as well, such as ESLint.

All features described in this blog post are part of the latest node2nix release (version 1.1.0) that can be obtained from the NPM registry and the Nixpkgs collection.

Besides a new release, node2nix is now also used to generate the expressions for the set of NPM packages included with the development and upcoming 16.09 versions of Nixpkgs.